Blud's done a good job with a specific case, I'll just skim through some more general things:
- hit/miss - both melee and spell - make sure you hit the "+" button to distinguish between miss, dodge and parry
- melee being parried - bad!
- any caster wanding - check their mana pot/gem usage under buffs and debuffs
- shammy totems - click on a shammy and then click on a totem will tell you how many times it was used (across the whole night, or across a single fight)
- buffs (castable, eatable and temporary weapon ones) are shown under buffs and debuffs, as are things like the debuff caused by leo's ww.
As to what are its limitations, basically everything it shows happens, however it doesnt necessarily show the whole story. A hunters dps much lower on leo for example, may be due to him getting demon'ed every time, tank dies on leo, might be because a healer was healing someone who got ww'ed, melee dps low on lurker - he mght have run out to avoid every whirl (unlike the others) or may have been threat-capped. Some of these you can investigate using wws, but take some looking others are not possible to fully explore.
I'd start with the more general things